A common organic EL device comprises an anode that is provided on a substrate and is made of an ITO or the like, an organic layer provided on the anode and a cathode provided on the top of it. The organic layer has a configuration in which a hole-injecting layer, a hole-transporting layer and an emitting layer are stacked in sequence from the side of the anode, for example. In an organic EL device having such a configuration, light that is emitted when electrons injected from a cathode and holes injected from an anode are recombined in an emitting layer is outcoupled from the substrate.
In general, the life of an organic EL device is determined by the amount of injected charges. Therefore, there is a problem that, if the driving current is increased in order to increase the initial luminance, the life is shortened with an increase in driving current.
In order to solve the problem, it is required to increase the luminance without changing the driving current (i.e. to increase the efficiency) or to realize a device configuration that can obtain the equivalent luminance even if the driving current is lowered.
As the above-mentioned device configuration, a configuration (MPE device) in which a plurality of emitting units each comprising organic layers that at least include an emitting layer are stacked between an anode and a cathode through an insulating charge-generating layer has been proposed. Here, the charge-generating layer is a layer that serves, at the time of application of a voltage, to inject holes to an emitting unit that is provided on the side nearer to the cathode of the charge-generating layer, and to inject electrons to an emitting unit that is provided on the side nearer to the anode of the charge-generating layer.
In a stacked organic EL device obtained by stacking emitting units through the charge-generating layer, it is believed as follows. That is, when two emitting units are stacked, the luminance [cd/A] can be doubled ideally with the luminous efficiency [lm/W] being unchanged, and when three emitting units are stacked, the luminance can be tripled ideally with the luminous efficiency [lm/W] being unchanged.
As for an MPE device, in Patent Documents 1 and 3, for example, a device using a transparent conductor (e.g. ITO, IZO) in a charge-generating layer is disclosed.
In Patent Document 2, a device using vanadium oxide (V2O5) or rhenium heptoxide (Re2O7) in a charge-generating layer is disclosed. Patent Document 4 discloses a device using a metal oxide such as molybdenum oxide (MoO3) or a metal salt such as iron chloride (FeCl3) is used in a charge-generating layer. Patent Document 5 discloses a device using a combination of an N-dope layer and a P-dope layer in a charge-generating layer. Patent Document 6 discloses a device using a phthalocyanine compound in a charge-generating layer. Patent Document 7 discloses a device using hexazatriphenylene (HAT) and an electron-accepting organic substance disclosed in Patent Document 2 (e.g. F4TCNQ) in a charge-generating layer.
As mentioned above, although various materials are used in a charge-generating layer, there are some problems associated with conventional charge-generating layers.
Specifically, since a high temperature is required when an inorganic substance such as a metal oxide is formed into a film by deposition, the efficiency of film-forming process is lowered, resulting in poor mass productivity.
Further, since a transparent conductor such as an ITO has a high electric conductivity, current leakage between pixels through a charge-generating layer may occur. Therefore, when a desired pixel is caused to emit light, adjacent pixels may also emit light. This phenomenon becomes a problem especially in the case of a display in which an organic EL device is allowed to emit white color and each of RGB colors is outcoupled through a color filter provided on the device. That is, since color purity is significantly lowered due to occurrence of color mixing by emission of adjacent pixels, color reproducibility is lowered.
In addition, damage to organic layers serving as underlying layers by generated plasma particles is concerned at the time of forming a transparent conductor by sputtering or the like.
On the other hand, when an organic compound is used, in general, the temperature at the time of film formation is low and no plasma particles are generated. Therefore, the efficiency of film formation process or mass productivity is excellent. When an electron-accepting organic compound is used, as shown in Patent Document 7, in particular, a device having excellent properties such as efficiency, voltage and life can be obtained when HAT is used. However, since HAT itself has high conductivity, as in the case of the above-mentioned transparent conductor, current leakage between pixels through a charge-generating layer may occur.